The present invention relates to the field of mass storage devices. More particularly, this invention relates to a method of screening disc drives for various harmonic frequencies.
One key component of any computer system is a device to store data. Computer systems have many different places where data can be stored. One common place for storing massive amounts of data in a computer system is on a disc drive. The most basic parts of a disc drive are an information storage disc that is rotated, an actuator that moves a transducer head to various locations over the disc, and electrical circuitry that is used to write and read data to and from the disc. The disc drive also includes circuitry for encoding data so that it can be successfully retrieved and written to the disc surface. A microprocessor controls most of the operations of the disc drive as well as passing the data back to the requesting computer and taking data from a requesting computer for storing to the disc.
The transducer head is typically placed on a small ceramic block, also referred to as a slider, that is aerodynamically designed so that it flies over the disc. The slider is passed over the disc in a transducing relationship with the disc. Most sliders have an air-bearing surface (xe2x80x9cABSxe2x80x9d) which includes rails and a cavity between the rails. When the disc rotates, air is dragged between the rails and the disc surface causing pressure, which forces the head away from the disc. At the same time, the air rushing past the cavity or depression in the air bearing surface produces a negative pressure area. The negative pressure or suction counteracts the pressure produced at the rails. The slider is also attached to a load spring which produces a force on the slider directed toward the disc surface. The various forces equilibrate so the slider flies over the surface of the disc at a particular desired fly height. The fly height is the distance between the disc surface and the transducing head, which is typically the thickness of the air lubrication film. This film eliminates the friction and resulting wear that would occur if the transducing head and disc were in mechanical contact during disc rotation. In some disc drives, the slider passes through a layer of lubricant rather than flying over the surface of the disc.
Information representative of data is stored on the surface of the storage disc. Disc drive systems read and write information stored on tracks on storage discs. Transducers, in the form of read/write heads attached to the sliders, located on both sides of the storage disc, read and write information on the storage discs when the transducers are accurately positioned over one of the designated tracks on the surface of the storage disc. The transducer is also said to be moved to a target track. As the storage disc spins and the read/write head is accurately positioned above a target track, the read/write head can store data onto a track by writing information representative of data onto the storage disc. Similarly, reading data on a storage disc is accomplished by positioning the read/write head above a target track and reading the stored material on the storage disc. To write on or read from different tracks, the read/write head is moved radially across the tracks to a selected target track. The data is divided or grouped together on the tracks. In some disc drives, the tracks are a multiplicity of concentric circular tracks. In other disc drives, a continuous spiral is one track on one side of disc drive. Servo feedback information is used to accurately locate the transducer head. The actuator assembly is moved to the required position and held very accurately during a read or write operation using the servo information.
The actuator is rotatably attached to a shaft via a bearing cartridge which generally includes one or more sets of ball bearings. The shaft is attached to the base and may be attached to the top cover of the disc drive. A yoke is attached to the actuator. The voice coil is attached to the yoke at one end of the rotary actuator. The voice coil is part of a voice coil motor which is used to rotate the actuator and the attached transducer or transducers. A permanent magnet is attached to the base and cover of the disc drive. The voice coil motor which drives the rotary actuator comprises the voice coil and the permanent magnet. The voice coil is attached to the rotary actuator and the permanent magnet is fixed on the base. A yoke is generally used to attach the permanent magnet to the base and to direct the flux of the permanent magnet. Since the voice coil sandwiched between the magnet and yoke assembly is subjected to magnetic fields, electricity can be applied to the voice coil to drive it so as to position the transducers at a target track.
Controlling the movement of the actuator and attached transducing heads is achieved with a closed loop servo system. U.S. Pat. No. 5,262,907 issued to Duffy et al., and assigned to the assignee of the present invention details an example of such a closed loop servo system. In such a system, position or servo information is prerecorded on at least one surface of one of the discs. The servo system can be either a xe2x80x9cdedicatedxe2x80x9d servo system, in which one entire disc surface in a disc stack is prerecorded with the servo information and a dedicated servo head is used to constantly read the servo information, or an xe2x80x9cembeddedxe2x80x9d servo system, in which servo information is interleaved with user data and intermittently read by the same heads used to read and write the user data.
With either a dedicated or embedded servo system, it is common that the servo circuitry produce a servo position error signal (PES) which is indicative of the position of the head relative to the center of a track. The identity of the particular track, as well as other information relating to the circumferential position of the head on the track, is included, along with other information, in the prerecorded servo information. Thus, when the heads are following a desired track, the PES is essentially at a zero value. The PES is fed back to circuitry used to control current through the coil of the actuator. Any tendency of the heads to deviate from true track center causes the PES to change from its zero value. The PES is a bipolar analog signal, meaning that deviation of the head position away from track center in a first direction will produce a PES of a first polarity, while movement of the heads off track center in the opposite direction will produce an PES of the opposite polarity, and the greater the distance of the head from track center, the greater the magnitude of the PES signal. It should be noted that the PES signal relates to each track centerline, and, as such, when the actuator is seeking from one track to another, the PES signal switches from maximum offset value from a first track in a first direction to maximum offset value from a second track in the opposite direction as the moving head passes the midpoint between the first and second tracks.
In the manufacture of disc drives, it is not unusual for tens of thousands of disc drive units to be fabricated daily. With such high numbers of disc drives being made, it is apparent that a certain number of units will fail to meet the design specifications, due to faulty components, improper assembly, contamination, and other elements familiar to those of skill in the art. While every effort is made by disc drive manufacturers to minimize these defective units and assembly errors, a small percentage of defective units will occur. When the defect is introduced into the unit at an early stage in the manufacturing process, the fault may not be detected until a much later stage of the process. Such a delay in the detection of defective assemblies can result in a significant amount of labor costs when taken over the large numbers of units being manufactured.
It has been found that several mechanical defects that can commonly be introduced into the assembly of a disc drive can be closely correlated to the introduction of susceptibility of the unit to resonances at fixed xe2x80x9cmarkerxe2x80x9d frequencies. This correlation has come about empirically with the experience of building hundreds of thousands of identical products. With this knowledge, it follows that if the disc drive units can be tested for resonance at the marker frequencies, early detection of the manufacturing defects is possible.
It has been found that resonant frequencies in a mechanical structure can sometimes be identified through the use of a frequency analyzer which, once properly connected to the structure to be tested, injects energy at a selected frequency and then evaluates the structure for gain in the energy which would be indicative of resonance. While the use of a frequency analyzer as an engineering diagnostic tool is well known in the industry, it does have several drawbacks which make such use impractical for large-scale implementation in disc drive manufacturing test operations. Firstly, a frequency analyzer is a complex and expensive piece of diagnostic test equipment, costing several thousand dollars per unit. In a manufacturing environment producing tens of thousands of units per day, a large number of frequency analyzers would be needed in order to provide adequate test capability for the quantity of drives being manufactured, resulting in economically prohibitive capital costs for the manufacturer. Secondly, connecting an analyzer to each structure to be tested and performing the test would require both an operator and a significant amount of time, two elements antithetical to such a high volume production environment. Thirdly, the implementation of automated test result reporting and evaluation with such discrete test equipment would be difficult and resource intensive.
During manufacturing, resonance screening may include analyzing the PES using digital fourier transform (DFT) to analyze the frequency components of the PES. This requires computer time and may slow down the testing portion of the manufacturing process. In addition, such a test may not identify all the potential resonances since using the DFT on a data signal removes phase information. Analysis using DFT on the PES also does not account for shifts in the resonant frequency that may occur as a result of an increase in temperature within the disc drive as it moves to an operating temperature or that may occur as a result of differences in the mechanical components that make up the actuator assembly.
It has also been found that testing for sympathetic resonances in a structure can be accomplished by mounting the unit to be tested to a vibration table, and then injecting either sinusoidal or random vibration energy into the unit during operation and then monitoring for resonant frequencies using suitable test equipment. Again, such a method, although useful during development of a disc drive, would be economically impractical for implementation during large scale manufacture due to capital equipment and resource requirements.
It would, therefore, be desirable to provide a method and apparatus for testing for mechanical defects in disc drives by detecting resonances at corresponding marker frequencies, and culling out units failing the test procedure for repair or remanufacture, while allowing passing units to continue onward in the manufacturing process. It would also be preferable if the test methodology involved a minimum of cost, both in human operator time and capital investment.
What is needed is a method and apparatus to quickly screen disc drives for various frequencies, including frequency around a calculated frequency. There is also a need for a screening test that will determine resonant frequencies for different situations, such as when a disc drive is at an operating temperature or when the mechanical components cause a shift in the frequency. There is also a need for screening test which is quick and which can be easily incorporated into the current manufacturing and testing process. There is still a further need for a screening test that will indicate the amount of gain margin for the resonance.
A disc drive 100 includes abase 112 and a disc 134 rotatably attached to the base 112. The disc drive 100 also includes an actuator assembly 120 rotatably attached to said base 112 and a device for moving the actuator assembly. The actuator assembly 120 includes an actuator arm 123 and a transducer head 150 in a transducing relationship with respect to the disc 134. The transducer 150 is attached to the actuator arm 126. A method of screening disc drives for harmonic resonant frequencies includes sampling the position error signal at a track location in the disc drive, and determining the velocity of the position error signal from the sample of the position error signal. The velocity of the position error signal sample is divided by the position error signal to produce a quotient. The quotient is compared to a selected quotient threshold value to determine the type of a harmonic in the disc drive. The method further includes storing the track number; and identifying the track as a track not to receive information when the quotient indicates a defect in the disc drive, and preventing the track from receiving information when the quotient is greater than selected value. The sampling the position error signal step includes reading the position error signal using a servo control loop. The determining the velocity of the position error signal step includes taking the derivative of the position error signal with respect to time. The mechanical defect is tagged as related to high frequency harmonics when the value of quotient is greater than the threshold value. The dominant harmonic frequency associated with the mechanical defect is calculated. The mechanical defect is tagged as related to low frequency harmonics when the value of quotient is less than or equal to the threshold value and the dominant harmonic frequency associated with the mechanical defect is calculated.
The step of calculating the dominant harmonic frequency associated with the mechanical defect may include using the quotient in determining the slope of a line associated with the dominant frequency. From the quotient used to determine the slope of a line associated with the dominant a constant multiplier for the quotient can be calculated that produces a best fit to a set of samples at various frequencies. The calculating step can include calculating a constant to add to the quantity of the constant multiplier and the quotient to produce a function best fit to a set of samples at various frequencies.
In addition, a disc drive device includes a base, a disc rotatably attached to the base and an actuator arm for carrying a transducer head in a transducing relation with respect to the disc. The disc drive has a disc drive controller which is coupled to the actuator arm. The disc drive controller further includes a servo controller also coupled to the actuator arm. The servo controller monitors a position error signal in order to follow a track on the disc drive. The disc drive also includes a microprocessor for determining the dominant frequency in the disc drive. The microprocessor samples the position error signal, determines the rate of change of the position error signal with respect to time, calculates the quotient of the rate of change of the position error signal with respect to time and the position error signal, and compares the quotient to a selected threshold. The microprocessor uses the quotient to determine a simulation of a dominant frequency. The microprocessor monitors the position error signal for resonant frequencies within a selected range of resonant frequencies. In some embodiments, the microprocessor monitors the position error signal for resonant frequencies within a selected range of resonant frequencies which includes a dominant frequency. In other embodiments, the microprocessor stores the particular locations on the disc where resonant frequencies occur. In still other embodiments, the microprocessor monitors the position error signal for resonant frequencies within a selected range of resonant frequencies at a particular location on the disc which includes a dominant frequency, and fails the disc when resonant frequencies are identified.
Advantageously, the inventive method and apparatus quickly determines one or more dominant frequencies which can be used to screen disc drives for mechanical defects. More particularly, the method disclosed assures that the proper frequency is assigned when one of two frequency occurrences may satisfy a condition using other methods. The method can be used to quickly approximate the dominant frequency for the disc drive and specific tracks on the disc drive. The harmonic frequency of failed tracks are logged and displayed for failure analysis and the track or tracks that fail the test are passed over for data storage. The end result is a more reliable disc drive having less read errors.